The present invention relates to a novel lower density polyethylene, having a multimodal comonomer distribution, and products obtained from use of such polyethylene inter alia for manufacturing extrudated or blown films. Surprisingly, the LLDPE composition of the present invention displays drastically enhanced mechanical impact resistance as well as excellent processing properties, allowing of obviating the addition of processing aids, notably of fluoroelastomers, in film processing.
Polyolefine films made from metallocene-derived LLDPE have become state-of-the-art for foils or films used for packaging goods, due to their good optical properties and sealing strength. However, good processability is not a stronghold of LLDPE films in contrast.
U.S. Pat. No. 5,420,220/Mobil Oil describes a monomodal LLDPE polymer of 0.918 g/cm3 having good dart drop impact strength of about 800 g and good optical properties with a haze value of 5-7, but has very low melt flow index (@2.16 kg) of only 1 g/10 min (and a melt flow ratio MFR21/2=17, MWD=2.6). The monomodal product is polymerized by catalysis with bis(n-butylcyclopentadienyl) zirconium dichloride in a fluidized bed reactor. Whilst films may be manufactured from such product, given the low melt flow rates, film extrusion of such LLDPE requires elevated working pressure and suffers from risk of melt fracture, necessitating to add film processing auxiliaries which is technically undesirable and defies certain production needs, e.g. for food or pharmaceutical packaging products. The processing additives are easily extractable and are deemed hazardous to health and environment.
Often, it is sought to improve the processing properties of such material by adding some amount of more broadly distributed, high density polymer such as classic HDPE grades obtained with Ziegler catalysts.
WO 2001/098409/Univation describes bilayered films made from a blend of homopolymeric HDPE and of metallocene-derived, narrowly distributed VLDPE having a density of from 0.89 to 0.915 g/cm3 in a mixing ration of 20:80, a MWD=Mw/Mn of from 2.0 to 3.0, a CDBI of 50 to 85% the VLDPE being TREF-biomodal, and comparing them to similar, non-blended films made from either one of said components. Despite being bilayered, the dart drop impact strength obtained was only 634 g/mil concomitant with acceptable, but not superior haze values of about 10 and a somewhat inferior gloss.
WO2005/061614/Univation again describes blends of metallocene-produced LLDPE with 2 to 10% (w/w) of different HDPE grades, yielding polymer compositions of a density of from 0.921-0.924 g/cm3 having a melt flow index (@2.16 kg) of about 1.1 g/10 min and a very low dart drop impact of 166 to 318 g only; in fact, even for blends made with HD-LDPE instead of HDPE, the loss of dart drop as compared to the isolated metallocene product usually amounted to 50% or more. At least for some isolated HDPE grades, a good haze of below 10% was reported, however, not balanced by a good dart drop. In summary, it was not achieved to preserve the superior dart drop properties of the metallocene product in the blended composition.
EP-1333 044 B1/Borealis describes a cascaded reactor process firstly synthesizing a high density, low molecular weight ethylene-1-hexene copolymer in a first and second reactor, and finally blending such second product having a density of 0.949 g/cm3 and a melt flow index (@2.16 kg) of 310 g/10 min. being indicative of a comparatively low weight and low viscosity at shear, with a high-molecular weight ethylene-1-buten-copolymer synthesized in a third reactor. A Ziegler-Natta-catalyst was used throughout the reactor cascade. The ensuing VLDPE/HDPE blend had a high load melt flow index (@21.6 kg) of 27 g/10 min. and a melt flow rate MFR of 27, indicative of a strongly increased viscosity at a total density of 0.923 g/cm3. The optical properties of such product were extremely poor, dart drop however amounted to >1700 g. The high viscosity and inferior optical properties however, do not compensate for the superior dart drop impact resistance displayed by the film prepared from such blend.